Tunable transmitter with Mach-Zehnder modulator

Abstract

An improved wavelength tunable transmitter comprises a waveguide grating router coupled to a truncated Mach-Zehnder interferometric modulator. Light from the second order Brillouin zones of a waveguide grating router having substantially the same phase are captured and coupled to first and second transmission paths comprising the truncated Mach-Zehnder interferometer, the transmission paths then being altered relative to each other and recombined to produce a modulated output signal. With this configuration, a compact tunable transmitter is provided exhibiting bandwidth and chirp control features comparable with those of a traditional Mach-Zehnder interferometer.

I claim: 
     
       1. A wavelength tunable transmitter comprising: (a) a waveguide grating router comprising (i) a source of incident plane wave of light having an input wavelength λ i  and a phase, and   (ii) at least a first free space region configured to receive the input light from the source and generate first order and a pair of second order Brillouin zones having the same wavelength λ i , the pair of second order Brillouin zones defining a pair of waveguide outputs having substantially the same phase, and     (b) a truncated Mach-Zehnder interferometer comprising two incoming transmission paths for receiving the pair of outputs, a means for altering the phase of one transmission path relative to the other, and a means for combining the transmission paths to a single output having a predetermined wavelength λ i , wherein one of each of the pair of outputs in the second order Brillouin zones is coupled to one of the transmission paths of the interferometer, whereby the second order Brillouin zones are employed as an output from the waveguide router to enable high speed modulation of the predetermined wavelength λ i  at the single output, and   wherein the source comprises a series of amplifiers so that selective activation of one of the series of amplifiers will determine the predetermined wavelength λ i  at the single output.   
     
     
       2. The tunable transmitter according to claim 1, in which the waveguide router further comprises a second free space region connected to the first free space region by a plurality of unequal length grating arms. 
     
     
       3. The tunable transmitter according to claim 2 further comprising a single amplifier coupled to the second free space region. 
     
     
       4. The tunable transmitter according to claim 1, in which the truncated Mach-Zehnder interferometer comprises a substrate having the two transmission paths disposed thereon, and the means for altering the phase of one transmission path comprises a region disposed on the substrate along at least one of the transmission paths having a refractive index different from that of the substrate. 
     
     
       5. The tunable transmitter according to claim 1, in which the means for altering the optical properties of one transmission path comprises the transmission paths having differing lengths. 
     
     
       6. The tunable transmitter of claim 1 in which the waveguide grating router and the truncated Mach-Zehnder interferometer are monolithically integrated on a substrate. 
     
     
       7. An optical fiber communication system including the tunable transmitter of claim 6 and further comprising a receiver and at least one fiber, the at least one fiber defining an optical path connecting the transmitter and the receiver. 
     
     
       8. A wavelength tunable transmitter comprising: (a) a waveguide router comprising a series of tuning amplifiers defining a source of input light i , a first free space region for receiving light from the source, a second free space region, and a plurality of waveguide grating arms connecting the first and second free space regions, wherein selective activation of at least one of the series of amplifiers will determine the wavelength λ i  of the input light;   wherein the first and the second free space regions generate first order and a pair of second order Brillouin zones having the same wavelength λ i  the pair of second order Brillouin zones being transmitted over the plurality of grating arms between the first and second free space regions, and the second free space region of the waveguide router emitting a pair of waveguide outputs in the second order Brillouin zones, the pair of outputs modes defining a first waveguide output and a second waveguide output having substantially the same phase,   (b) a truncated Mach-Zehnder interferometer comprising two incoming transmission paths for receiving the first and second waveguide outputs, a means for altering the phase of one transmission path relative to the other, and a means for combining the transmission paths to a single output,   wherein the first and second waveguide outputs of the waveguide router are coupled to the truncated Mach-Zehnder interferometer for altering the first waveguide output relative to the second waveguide output and for recombining the first and second waveguide outputs to produce the output signal having a predetermined wavelength λ i ,   whereby light in the second order Brillouin zones is employed as an output from the waveguide router to enable high speed modulation of the wavelength λ i  at the single output by selective activation of at least one of the series of amplifiers.   
     
     
       9. The transmitter of claim 8 having channel spacings of about from 0 GHz to 500 GHz. 
     
     
       10. The transmitter of claim 9 in which the waveguide grating router and the truncated Mach-Zehnder interferometer are monolithically integrated on a laser cavity. 
     
     
       11. An optical communications system comprising the transmitter of claim 10 mounted on an optical subassembly. 
     
     
       12. A method of modulating an optical signal generated by a laser, the method comprising the steps of: providing a waveguide router for receiving an optical signal generated by the laser having a wavelength λ i  and a phase, wherein the waveguide router splits the optical signal into first order and second order Brillouin zones and thereby produces a pair of waveguide outputs in the second order Brillouin zones having substantially the same phase;   coupling one of the pair of waveguide outputs to a first transmission path of a Mach-Zehnder interferometer and the other of the pair of waveguide outputs to a second transmission path of a Mach-Zehnder interferometer;   altering the phase of one transmission path relative to the other; and   combining the first and second transmission paths to a single output signal so that the interference caused by combining the transmission paths having differing optical properties produces a modulated signal.   
     
     
       13. The method of claim 12, further providing the steps of: coupling a plurality of amplifiers to the waveguide router and tuning the CW wavelength of the signal generated by the laser by selectively activating one or more of the plurality of amplifiers.   
     
     
       14. The method of claim 12, further providing the step of tuning the CW wavelength of the signal generated by the laser by altering the temperature of the device.